Discs
are the pads between the vertebrae of the spine. There are twenty-four
mobile vertebrae and five that are fused together to form the sacrum.
Discs are vulnerable to injury because they have the difficult job
of being flexible enough to move, but strong enough to take enormous
stress.

The discs between the lower lumbar vertebrae are the most commonly
injured because they support the weight of the entire upper body.
The structure of the disc determines the extent of injury, if one
occurs.

Disc herniation, rupture,
protrusion, and extrusion are all terms
that describe this injury. There are subtle differences between
these terms, but they all refer to a tear in the outer ring, which
allows some of the inner core to escape. A herniation occurs when
a portion of the soft inner core (nucleus pulposis)
escapes through the outer ring (annulus).

The disc may look like a solid piece
of padding between the vertebrae, but it is far more complex. Discs
are composed of two parts that work together to carry the load.

The strong outer ring or annulus,
is firmly attached to the vertebrae above and below the disc.
The ring has strong crosshatched fibers much like the steel belts
of a car tire. This outer ring is about 1/4 inch thick, and by
itself, doesn't have much compressive strength. If this ring were
empty, it would collapse when weight was applied to the bone above.
The outer ring holds the nucleus pulposis in place.

The nucleus pulposis is located
inside the annulus. This is a soft gelatin-like cartilage with
a very high water content which provides cushioning while evenly
supporting the weight of the body. This cartilage transmits the
load from bone to bone.

There is a delicate balance between the forces trying to push
the gelatin-like nucleus out to the sides and the tension of the
outer ring holding it in place.

The disc has two functions:

it transmits the weight of the body
from the bone above to the bone below.

it allows the spine to be flexible.

The lumbar (lower back)
discs must support the weight of the entire upper body plus whatever
is picked up or carried. Because the body's center of gravity is
in front of the discs, loads on the disc are increased by leverage,
much like a nutcracker multiplies force on a nut. Research has
shown that picking up a 20 pound weight may increase pressures inside
the disc by more than eight times the body's weight. The job
of the disc is to transmit this load efficiently and evenly to the
vertebra below. The gelatin-like nucleus pulposis distributes the
stress evenly across the surfaces of the adjacent bones. The combination
of soft inner core and strong outer ring also allows maximum motion
between the bone segments. Both of these functions are severely
tested in the lumbar spine, where enormous loads frequently occur
and physical activities demand wide ranges of motion.

What is a herniated
lumbar disc?

The outer ring of the disc is subject
to continuous stresses that sometimes cause small tears to develop.
If one of these tears is large enough and the stresses inside
the disc strong enough, some of the pulposis can leak through
the tear. The disc is most vulnerable to injury at the weakest
parts of the outer ring, where nerves pass through the spinal
canal and exit toward the legs. If disc material compresses
one of these nerves, there can be pain both in the back and
down the leg . Pain down the leg is often called sciatica,because
the most frequently pinched nerves form the sciatic nerve,
which runs down the back of the thigh to the lower leg.

A disc can herniate due to either
a sudden load or an accumulation of minor injuries over time.
At least 50% of patients can't recall a specific event that
caused the ruptured disc. When an event can be identified, it
often involved lifting and twisting; activities that place severe
stresses on the disc.
Discs are named for the bones above and below them. For example,
the L4-5 disc is between the 4th and 5th lumbar bones. The L5-S1
disc is between the 5th lumbar and 1st sacral bones. These
two discs do the most work and are the most frequently injured.
Paralysis rarely occurs with disc herniation. The spinal
cord proper ends at the first lumbar vertebra (L1), well above
these frequently injured discs.
Other conditions that cause back and leg pain can mimic a ruptured
lumbar disc. Some of these conditions are:

arthritis of the hip or knee area.

pulled muscles in the back (lumbar strain).

pulled muscles in the leg and other conditions that
cause irritation of the spinal nerves.